The present invention is directed to apparatus for lying elongated articles in conduits such as for laying electrical cables in underground ducts.
Electric utilities lay large quantities of cables in underground ducts. Generally three cables are laid simultaneously for 3-phase electrical power. A typical duct has an internal diameter of about 4 to 6 inches, and the cable diameter typically is about 13/8 inches for 350,000 circular mil cable, and about 13/4 inches for 750,000 and 1,000,000 circular mil cable.
A common method for handling underground cable is the use of single reels with a single cable wound on each reel. The problem with this technique is difficulty in dispensing cable from three reels simultaneously into the duct and pulling long lengths of cable. Further, massive equipment is required. Method and apparatus for laying cable with this technique are described in U.S. Pat. Nos. 2,498,834, 2,532,504, 3,363,879, and 4,101,114, the disclosures of which are incorporated herein by this reference.
In order to overcome these problems and lay three cables simultaneously, cable manufacturers provide three lengths of cables helically twisted about each other into what is commonly referred to as a "triplex" cable. To lay the triplex cable, it is pulled through the conduit by means of a rope. At the feed end of the conduit, a split bell is provided for guiding the triplex. The bell is conventionally provided with a grease-fitting for radially inward introduction of lubricant to aid in pulling the triplex cable through the conduit.
There are many problems with this type of system. For example, it is difficult to pull the triplex cable through a conduit because the lubricant is applied only on the exterior surface of the cables. The high pulling forces required can stretch the cables, resulting in broken strands and line discontinuities. Another disadvantage of triplexing is that it is expensive, costing about 15 to 20 cents per linear foot.
A further disadvantage of the triplex cable system is that there can be substantial wastage of cable. If 600 feet of triplex are ordered, but only 500 feet are actually required, there are 100 feet of the triplex cable left over. This leftover triplex cable has little, if any use, because it cannot be splied to other triplex cable because a large splice cannot be pulled through a conventional underground duct.
Another problem with triplex cable is that most manufacturers supply it only in lengths up to about 900 feet, because that is the maximum that can be placed on a single reel. Therefore, for lengths longer than 900 feet, it is necessary to build splicing vaults into the underground duct system. The splicing vaults are expensive to construct, the labor for forming the splices is expensive, and the splices are often a source of line discontinuities.
In view of the problems associated with pulling triplex cable through a duct, it is evident that there is a need for a system that allows three cables to be laid in a duct easily, inexpensively, and with less waste cable.